The use of programmable array logic for implementing separate random logic functions as well as functions within microcomputers is well known in the prior art. U.S. Pat. Nos. 3,983,538 and 4,044,338 are examples of array logic which can implement either random logic or sequential logic. U.S. Pat. No. 4,044,338 includes interaction with a main memory. Most of the programmed logic arrays in use at the present time are permanently programmed at the time of manufacture by selective etching of metal connections or other similar techniques. Permanently programmed programmable logic arrays offer very dense logic implementation because fewer switching devices are required.
In recent years, reprogrammable logic arrays have been manufactured using some of the techniques used for programmable read/only memories. U.S. Pat. No. 4,041,459 is an example of a programmable logic array into which a logic pattern can be electronically written and stored in the metal insulation of insulated semiconductor storage transitors comprising the switching elements of the AND array and the OR array.
Accordingly it is known in the prior art to implement various logic functions in programmable logic arrays and it has further become known to electronically modify the logic in a programmable logic array, primarily to reduce the cost of making engineering changes.
A disadvantage of present programmable logic array systems is that each logical function implemented in the AND and OR arrays requires array space, independent of its relative importance measured in terms of activity or frequency of use.